Environmental and regulatory initiatives are requiring ever-lower levels of both sulfur and aromatics in distillate fuels. For example, proposed sulfur limits for distillate fuels to be marketed in the European Union for the year 2005 is 50 wppm or less. There are also regulations that will require lower levels of total aromatics in hydrocarbons and, more specifically, to lower levels of multi-ring aromatics found in distillate fuels and heavier hydrocarbon products. Further, the maximum allowable aromatics level for U.S. on-road diesel, CARB reference diesel, and Swedish Class I diesel are 35, 10 and 5 vol. %, respectively. Further, the CARB and Swedish Class I diesel fuel regulations allow no more than 1.4 and 0.02 vol. % polyaromatics, respectively. Consequently, much work is presently being done in the hydrotreating art because of these proposed regulations.
Hydrotreating, or in the case of sulfur removal, hydrodesulfurization, is well known in the art and typically requires treating the petroleum streams with hydrogen in the presence of a supported catalyst at hydrotreating conditions. The catalyst is usually comprised of a Group VI metal with one or more Group VIII metals as promoters on a refractory support, such as alumina. Hydrotreating catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally contain molybdenum or tungsten on alumina promoted with a metal such as cobalt, nickel, iron, or a combination thereof. Cobalt promoted molybdenum on alumina catalysts are most widely used when the limiting specifications are hydrodesulfurization. Nickel promoted molybdenum on alumina catalysts are the most widely used for hydrodenitrogenation, partial aromatic saturation, as well as hydrodesulfurization.
The ability to modify the nanostructural morphology of supported hydrotreating catalysts provides a possible way to control their activity and selectivity. One of the important thrusts in hydrotreating catalyst research has been the realization over the last few years that a key synthesis tool for modifying nanostructure involves the incorporation of carbon into the sulfide structure. For example, U.S. Pat. No. 4,528,089 teaches that the use of carbon-containing catalyst precursors gave more active catalysts than catalysts prepared from sulfide precursors without organic groups. Use of organic impregnation aids in preparing oxide catalyst precursors has also been studied for some time (Kotter, M.; Riekeft, L.; Weyland, F.; Studies in Surface Science and Catalysis (1983), 16 (Prep. Catal. 3), 521-30 and U.S. Pat. No. 3,975,302.
While such catalysts have proven to be superior to more conventional hydrotreating catalysts, there still remains a need in the art for ever-more reactive and effective catalysts for removing heteroatoms, such as nitrogen and sulfur from hydrocarbon streams.